In recent years, there has been a dramatic increase in demand for batteries as energy sources with the technical development and increase in demand for mobile devices. As vehicles such as hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and electric vehicles (EV) have come into the spotlight as vehicles of the future, research on batteries capable of meeting various requirements has been conducted accordingly. In particular, research on a lithium secondary battery having a high energy density and also exhibiting excellent lifespan and cycle characteristics as a power source for such devices has been actively conducted.
A lithium secondary battery refers to a battery in which a non-aqueous electrolyte containing lithium ions is included in an electrode assembly that includes a cathode including a cathode active material enabling intercalation/deintercalation of lithium ions, an anode including an anode active material enabling intercalation/deintercalation of lithium ions, and a microporous separator interposed between the cathode and the anode.
The most important challenge in preparing anode slurry for preparing the anode is to increase dispersibility of the anode active material. A thickening agent serving as a dispersing agent is added to achieve sufficient dispersibility. In this case, when the thickening agent is added in a very small amount, the anode active material may not be sufficiently dispersed in the anode slurry, and thus large aggregates may be formed or slurry precipitation may occur. As a result, when a current collector is coated with the slurry, problems regarding filter plugging, surface defects of electrodes, and the like may occur. On the other hand, when the thickening agent is added in a very large amount, migration of lithium ions in the anode may be prevented, resulting in increased resistance of the anode. Therefore, the most important challenge is to minimize an amount of the thickening agent in order to maintain sufficient dispersibility and simultaneously reduce the resistance of the anode.
Accordingly, the present inventors have conducted research on carboxymethyl cellulose (CMC) as a thickening agent, found that an adsorption amount of CMC adsorbed onto an anode active material varies depending on physical properties of CMC, such as a degree of substitution or a molecular weight, compared changes in viscosity and precipitation of CMC while changing the physical properties and a blend amount of CMC, and finally prepared anode slurry having a reduced blend amount of CMC and simultaneously exhibiting excellent dispersibility, and a secondary battery having reduced resistance as well.